The present invention relates to a channel switching system and, more particularly, to a channel switching system for switching between a regular channel and a standby channel without bit error regardless of the presence of fading in a digital radio transmission channel.
In a conventional digital radio transmission circuit, a standby channel is assigned to a plurality of regular channels in consideration of operational availability of the regular channels and their equipment failure. When a given regular channel fails, it is switched to the standby channel. Problems of operational availability of the regular channels occur when maintenance and inspection are performed for the transmission lines of the regular channels and an anti-fading measure is required. In this case, a hitless channel switching system for switching the regular and standby channels has the most significance in respect of operational availability of a digital radio transmission circuit.
FIG. 1 is a block diagram showing the main parts of the transmitting and receiving sides of a digital radio transmission circuit to which a conventional channel switching system is applied. This transmission circuit has first, second, . . . mth regular channels. Referring to FIG. 1, digital multiplexed signals S44-1, S44-2, . . . S44-m supplied from a predetermined multiplexer/demultiplexer unit (not shown) are supplied to split circuits 38-1, 38-2, . . . 38-m, respectively. First split outputs are supplied to bipolar-to-unipolar (to be referred to as a B/U hereinafter) converters 41-1, 41-2, . . . 41-m of the respective channels, and second split outputs are supplied to corresponding switching circuits 39-1, 39-2, . . . 39-m. In normal operation, these digital multiplexed signals are converted from bipolar signals to unipolar signals by the B/U converters 41-1, 41-2, . . . 41-m. Transmitting signal processing circuits (XMTG SIG PROC CKTs) 42-1, 42-2, . . . 42-m insert frame sync bits and parity bits into the unipolar signals, respectively. The transmitting data are supplied to the receiving side through transmitting signal distribution circuits (XMTG SIG DISTR CKTs) 44-1, 44-2, . . . 44-m and a modulator/transmitter (MOD & XMTR) 45. A pilot signal from a pilot generator 40 arranged in the transmitting side is always transmitted to the receiving side through the switching circuits 39-1, 39-2, . . . 39-m, a B/U converter 41, a transmitting signal processing circuit 42, a transmitting signal switching circuit 43 and the modulator/transmitter 45.
At the receiving side, the digital multiplexed signals of the respective regular channels which are received and demodulated through a receiver/demodulator 46 are supplied to frame sync circuits (FRAME SYNC CKTs) 47-1, 7-2, . . . 47-m which detect the corresponding frame sync bits. The bits of the digital multiplexed signals which are added at the transmitting side are then separated therefrom by received signal processing circuits (RECD SIG PROC CKTs) 50-1, 50-2, . . . 50-m through received signal sync switching circuits (RECD SIG SYNC SWs) 49-1, 49-2, . . . 49-m. The separated signals are converted from the unipolar signals to bipolar signals by unipolar-to-bipolar (U/B) converters 51-1, 51-2, . . . 51-m. The bipolar signals are then supplied as digital multiplexed signals S45-1, S45-2, . . . S45-m to a multiplexer/demultiplexer unit through switching circuits (SWs) 52-1, 52-2, . . . 52-m. In the standby channel at the receiving side, the pilot signal from the receiver/demodulator 46 is supplied to a pilot detector (PILOT DET) 53 through a frame sync circuit (FRAME SYNC CKT) 47, a received signal distribution circuit (RECD SIG DISTR CKT) 48, a received signal processing circuit (RECD SIG PROC CKT) 50, a U/B converter 51 and the switching circuits 52-1, 52-2, . . . 52-m. The pilot signal is used to always monitor the standby channel while the regular channels are in operation so as to immediately switch between the regular and standby channels.
A case will be exemplified wherein a second regular channel is switched to the standby channel due to a circuit failure. When the first to mth regular channels are operated in the normal state, the digital multiplexed signals from the transmitting signal distribution circuits 44-1, 44-2, . . . 44-m are supplied to the modulator/transmitter 45 and the transmitting signal switching circuit 43 of the standby channel, as described above. When the second regular channel is replaced with the standby channel, an input to the transmitting signal switching circuit 43 is switched in accordance with a predetermined protocol. The digital multiplexed signal for the second regular channel is supplied in place of the pilot signal to the modulator/transmitter 45 of the standby channel and is transmitted to the receiving side. In this state, the digital multiplexed signal for the second regular channel is transmitted to the receiving side through two transmission circuits as the second regular and standby channels.
The digital multiplexed signal sent through the second regular channel is supplied to the received signal sync switching circuit 49-2 through the receiver/demodulator 46 and the frame sync circuit 47-2. Meanwhile, the multiplexed signal from the standby channel is supplied to the received signal sync switching circuit 49-2 through the receiver/demodulator 46, the frame sync circuit 47 and the received signal distribution circuit 48. The received signal sync switching circuit 49-2 compares the data bit information as the reference data of the regular system with the data bit of the digital multiplexed signal of the standby channel. When the data bits of the regular and standby channels match with each other, the regular channel is switched to the standby channel. The digital multiplexed signal through the standby channel is supplied to the multiplexer/demultiplexer unit through the received signal sync switching circuit 49-2, the received signal processing circuit 50-2, the U/B converter 51-2 and the switching circuit 52-2. A circuit break due to the failure of the second regular channel can be prevented.
When a failure occurs in the B/U converter 41-2, the transmitting signal processing circuit 42-2 or the transmitting signal distribution circuit 44-2 of the second channel at the transmitting side, or a failure occurs in the received signal sync switching circuit 49-2, the received signal processing circuit 50-2 or the U/B converter 51-2 of the regular channel at the receiving side, the switch circuit 39-2 is operated to supply the digital multiplexed signal S44-2 instead of the pilot signal to the B/U converter 41 of the standby channel. The signal is supplied to the receiving side through the transmitting signal processing circuit 42, the transmitting signal switching circuit 43 and the modulator/transmitter 45 corresponding to the standby channel. At the receiving side, the digital multiplexed signal sent through the standby channel is supplied to the switching circuit 52-2 through the receiver/demodulator 46, the frame sync circuit 47, the received signal distribution circuit 48, the received signal processing circuit 50 and the U/B converter 51. The resultant signal is supplied as the digital multiplexed signal S45-2 to the multiplexer/demultiplexer unit.
In the conventional channel switching system described above, the transmitting signal switching circuit is arranged in the standby channel between the transmitting regular and standby channels, and at the same time, the transmitting signal distribution circuits are arranged in the regular channels, respectively. The transmitting circuit arrangement is complex and large. Furthermore, control of switching between the regular and standby channels is functionally complex. Frame sync errors occur in the frame sync circuit of the standby channel at the receiving side and prolong the channel switching time, thereby degrading channel efficiency.